1. Field of the Invention
The present invention relates to an aluminum alloy plate for discs with improved platability and also to a process for producing the same. More particularly, it is concerned with an aluminum alloy plate for discs such as magnetic discs used as the recording media of computers and also with a process for their production. This alloy plate has, also, its outstanding properties at the time of surface preparation and plating, and provides a smoothly finished surface having a minimum of nodules and micropits after polishing.
2. Description of the Prior Art
An aluminum alloy has been the sole material for producing the substrate of magnetic discs used as the recording media of computers because it meets most requirements for the substrate lightness, non-magnetism, rigidity to permit high-speed revolution, excellent surface finish obtained by precision cutting or polishing, and a certain degree of corrosion resistance.
According to the conventional technology, the magnetic recording discs are provided with a magnetic film mainly by the coating method. However, this method is being replaced by the plating method or sputtering method which has recently been developed to perform high-density recording on magnetic discs. The substrate for magnetic discs designed for plating or sputtering undergoes plating with Ni-P. This substrate is made of AA5086 alloy (Al-Mg) in most cases; but it is also made of JIS 7075 alloy (Al-Zn-Mg) in some cases.
These conventional aluminum alloys have a disadvantage in that the substrates which have undergone Ni-P plating tend to have rough surfaces, because they contain many intermetallic compounds (such as Al-Fe, Al-Fe-Si, Al-Fe-Mn, and Mg-Si) which are larger than 10 .mu.m. These compounds are pulled off during cutting, polishing, and surface preparation for Ni-P plating, leaving small holes in the substrate surface.
The JIS 7057 alloy (Al-Zn-Mg) has an additional disadvantage attributable to its excessive content of Cu and Zn. These elements form a coarse intermetallic compound which is responsible for the surface roughness after Ni-P plating. Moreover, if the cooling rate during flat-baking is improper, internal stress is induced. This stress worsens the flatness of the aluminum plate.
As mentioned above, the conventional aluminum alloys for magnetic discs have a disadvantage in that the resulting discs tend to have rough surface and hence plating defects such as pits (small holes). It has been a common practice to eliminate this disadvantage by forming a comparatively thick plated layer (30-50 .mu.m). However, a thick plated layer is expensive. Thus, reducing the thickness of the plated coating is important for cost reduction and improved productivity.
Apart from reducing the thickness of the plated coating, it is also important to reduce pits and roughness in the pretreatment for plating. To this end, an attempt was made to reduce the size of intermetallic compounds by using highly pure raw material such as 99.9% or 99.99% purity aluminum. Unfortunately, it was found that an aluminum alloy made of high-purity raw aluminum makes plated the surface rougher and lowers the adhesion of plated the at layer.
The present inventors found that this is due to the following reason. When an aluminum alloy substrate undergoes zincate treatment as the pre-finishing for plating, the deposition of zinc is coarse and uneven because the high-purity raw aluminum contains a lesser amount of Fe. In order to address the above-mentioned problems and to meet many requirements for magnetic discs, the present inventors carried out an extensive study which led to the finding that the platability can be improved by adding Cu and Zn as the alloying elements. On the basis of this finding, the present inventors previously proposed an aluminum alloy for magnetic discs. (See Japanese Patent Publication No. 2018/1987.) This aluminum alloy has been put to practical use with good reputation.
There is an increasing demand for low cost magnetic discs, and this makes it more important than ever to reduce the thickness of Ni-P plating and to reduce the expense for polishing. In other words, it is important to obtain a smooth plating surface by reducing nodules (semispherical projections which occur on the disc surface during Ni-P plating) and also by reducing micropits (very small holes in the surface layer which have been ignored in the past).